1. Field of the Invention
The present invention relates generally to degaussing apparatus and, more particularly to a degaussing apparatus for use in television receivers, computer display devices or the like.
2. Description of the Prior Art
In television receivers, an electron beam is caused to misland on a phosphor 1 due to an influence of terrestrial magnetism as shown in FIG. 1. As a result, a color mis-registration or the like occurs. In order to avoid the color mis-registration, the television receiver incorporates therein a series circuit of a degaussing coil 5 and a posistor 4 as shown in FIG. 2. The degaussing coil 5 is located at the rear surface of a cathode ray tube (CRT), though not shown. When an AC power source 3 is coupled to this series circuit, there is flowed a current of sine wave whose envelope is gradually attenuated as shown in FIG. 3. That is, when the series circuit is actuated, the resistance value of the posistor 4 is small. However, the posistor 4 generates heats in accordance with a lapse of time and the resistance value thereof is increased gradually with the result that a flowing current through the posistor 4 is attenuated gradually. Then, no current is flowed at an equilibrium temperature.
By flowing such attenuating current to the degaussing coil 5, the magnetism can be transferred to metal covers (magnetic shields) mounted to the inside and outside of the CRT and a shadow mask (aperture grill) so as to cancel the terrestrial magnetism out.
The degaussing circuit incorporated within the television receiver can perform the degaussing operation immediately after the AC power source 3 is coupled thereto and cannot perform the degaussing operation at an arbitrary timing under the condition that it is still supplied with power. This degaussing operation must be carried out prior to various adjustments in the production line of television receivers, for example. Therefore, in such production line, in order to perform the degaussing operation at an arbitrary timing under the condition that the degaussing circuit is powered, such a degaussing apparatus is proposed, in which a television receiver body is provided with terminals 7 and an external posistor 8 can be coupled to the terminals 7 from the outside. When the external posistor 8 is coupled to the terminal 7 at a predetermined timing as described above, no current is substantially flowed to the incorporated posistor 4 because the impedance of the posistor 4 is already increased very much. Accordingly, if the external posistor 8 is coupled to the terminal 7, then the attenuating current shown in FIG. 3 is flowed through the external posistor 8 and hence the degaussing operation is carried out.
Further, a degaussing apparatus shown in FIG. 5 is known. In this example of the prior-art degaussing apparatus, a circuit 10 is coupled to the degaussing coil 5. As shown in FIG. 5, the circuit 10 comprises a diode bridge circuit 11 driven by a power transistor 12, a current detecting resistor 13 for detecting a current flowing through the power transistor 12, a low-pass filter 14 for smoothing the output of the current detecting resistor 13 and feeding the smoothed output back to a DC amplifier 15 and an analog attenuating waveform generator circuit 16 for supplying an analog attenuating waveform signal to the DC amplifier 15.
The attenuating waveform generator circuit 16 is constructed by a differentiating circuit or the like in an analog fashion and generates an analog attenuating waveform which is then fed to the DC amplifier 15. The DC amplifier 15 drives the power transistor 12 in response to the attenuating waveform input thereto from the attenuating waveform generator circuit 16 so that, when a positive half wave is supplied from the AC power source 3, for example, a current is flowed by way of a loop formed of the AC power source 3, the degaussing coil 5, the diode bridge circuit 11, the power transistor 12, the current detecting resistor 13, the diode bridge circuit 11 and the AC power source 3, in that order. Whereas, when a negative half wave is supplied from the AC power source 3, then a current is flowed by way of a loop formed of the AC power source 3, the diode bridge circuit 11, the power transistor 12, the current detecting resistor 13, the diode bridge circuit 11, the degaussing coil 5 and the AC power source 3, in that order.
The current detecting resistor 13 generates a voltage corresponding to a current flowing therethrough and outputs the same to the low-pass filter 14. The low-pass filter 14 smoothes the voltage input thereto from the current detecting resistor 13 and feeds the voltage thus smoothed back to the DC amplifier 15. The DC amplifier 15 drives the power transistor 12 in response to a difference between the analog attenuating waveform signal input thereto from the attenuating waveform generator circuit 16 and the signal input thereto from the low-pass filter 14, thereby a servo operation being effected so that the current flowing through the power transistor 12 becomes equal to the attenuating waveform generated from the attenuating waveform generator circuit 16. Accordingly, the attenuating waveform generating circuit 16 generates a waveform similar to the attenuating current flowing through the posistor 4, for thereby carrying out the degaussing operation.
However, the conventional degaussing apparatus shown in the example of FIG. 4 must prepare the external posistor 8 whose characteristic is the same as that of the posistor 4 incorporated within the television receiver. As a result, this conventional degaussing apparatus cannot cope with the production line in which many kinds of video apparatus such as television receivers or the like are produced.
Further, in the example of the conventional degaussing apparatus shown in FIG. 5, since the reference waveform generated by the attenuation waveform generating circuit 16 is the analog envelope waveform, it is impossible to prepare this analog envelope waveform for a wide variety of apparatus. Also, since this analog envelope waveform is different from a waveform which results from full-wave-rectifying an attenuation waveform flowing through the current detecting resistor 13 in actual practice, the low-pass filter 14 is required. As a consequence, the attenuation waveform flowing through the degaussing coil 5 in actual practice becomes different from the attenuation waveform flowing through the degaussing coil via the posistor 4. Furthermore, it is impossible to set the rush current, which is provided immediately after the degaussing operation is started, to a reference value previously set.
In addition, since the starting timing of the rush current is not specified in any of the above-mentioned two systems of the conventional degaussing apparatus, a reproducibility of degaussing effect is poor.